Emergency lighting system

ABSTRACT

An emergency lighting system that includes an electric discharge lamp and a DC-AC converter to supply emergency power to the lamp upon failure of the AC line voltage. A battery is charged with a constant current from the AC supply voltage. A transistor switching device connects the battery to the converter when a sensing device senses the absence of the AC voltage. A bias voltage that holds the switching device closed is derived from the battery via the emitter-collector path of the transistor switch. A given drop in the battery voltage is sensed to open the switching device to prevent an excessive current drain on the battery. The presence of the AC voltage is sensed to hold the switching device open and thereby prevent operation of the converter during normal operation of the lighting system.

United States Patent [191 Patel EMERGENCY LIGHTING SYSTEM [75] Inventor:Raoji Patel, Watertown, Mass.

[73] Assignee: North American Philips Corporation, New York, N.Y.

22 Filed: Apr. 16,1973

21 Appl. No.: 351,243

[52] U.S. Cl. 307/66, 315/86 [51] Int. Cl. H02j 9/06 [58] Field ofSearch 307/66, 64, 85, 86, 87,

307/56, 43; 315/86, DIG. 7, 171

[56] References Cited UNITED STATES PATENTS 3,684,891 8/1972 Sieron307/66 Primary ExaminerHerman Hohauser Attorney, Agent, or FirmFrank R.Trifari; Bernard Fra n z lilau V Sept. 3, 1974 [5 7 ABSTRACT Anemergency lighting system that includes an electric discharge lamp and aDC-AC converter to supply emergency power to the lamp upon failure ofthe AC line voltage. A battery is charged with a constant current fromthe AC supply voltage. A transistor switching device connects thebattery to the converter when a sensing device senses the absence of theAC voltage. A bias voltage that holds the switching device closed isderived from the battery via the emitter-collector path of thetransistor switch. A given drop in the battery voltage is sensed to openthe switching device to prevent an excessive current drain on thebattery. The presence of the AC voltage is sensed to hold the switchingdevice open and thereby prevent operation of the converter during normaloperation of the lighting system.

12 Claims, 1 Drawing Figure 3 BALLAST g l L 60 Hz AC PAIENIEBsEP sumEMERGENCY LIGHTING SYSTEM BACKGROUND OF THE INVENTION This inventionrelates to an improved form of electric lighting system moreparticularly to a high efficiency emergency lighting system thatutilizes electric discharge lamps and rechargeable batteries.

Electric power failures due to inclement weather conditions andequipment breakdowns have been a plague for many years. In winter, iceand snow on exposed high voltage lines may cause a line to rupture andproduce a blackout in the community serviced thereby. Hurricanes andother wind storms also cause power failures. More recently, widespreadareas have suffered blackouts due to over-loading of the generating ortransmission equipment. A power failure, no matter what the cause, mayvery well jeopardize human life and thus there are many installationswhich require some form of emergency lighting system that willautomatically come into operation upon the occurrence of a powerfailure. The high efficiency of a fluorescent lamp makes it especiallyvaluable for use in an emergency lighting system.

Many of the emergency lighting systems available on the market utilize arechargeable battery as the source of power for the system. Since thereis a finite limit on the length of time that a battery can power anillumination system, it is important that the system have a highefficiency. A partial solution to this problem is to use a highefficiency DC-AC converter. One of the problems of available emergencylighting systems using a converter is that the converter cannot beoperated without a load. If a rechargeable nickel cadmium battery isused, care must be taken to limit the charge current to a value thatwill not exceed the maximum overcharge current of the battery.

It is also a common practice to provide a completely separate selfcontained emergency lighting system including separate emergency lamps,a battery together with a circuit for charging same, and means forsensing an AC power failure along with means for automaticallyconnecting the battery to the emergency lamps when a power failureoccurs. This is an expensive and generally inefficient solution of theproblem in that it requires two separate illumination systems, only oneof which is in use at any given time. One solution to this problem isdescribed in US. Pat. No. 3,660,714 wherein a single lamp or group oflamps is used for both the normal AC operation of the lighting systemand for emergency operation using a battery as the power source.

The present invention provides a new and improved circuit configurationfor an emergency lighting system which overcomes many of theilluminations and disadvantages of the aforesaid known emergencylighting systems.

It is therefore an object of the present invention to provide a new adimproved emergency lighting system of simplified and economicalconstruction.

Another object of the invention is to provide novel circuitry which willlimit variations in the battery charge current due to AC line voltagevariations so that a charge current can be supplied which closelyapproaches the maximum limit of the battery overcharge current.

A further object of the invention is to provide a high efficiencyemergency lighting system that can deliver electric energy to thelighting system for a longer period than similar systems generallyavailable at this time.

Yet another object of the invention is to provide an emergency lightingsystem wherein the battery does not supply power to the converter whenthe AC line voltage is present, but automatically supplies such powerupon a failure of the AC voltage.

A further object of the invention is to provide a new and improvedemergency lighting system having means for preventing a reversal in thebattery polarity by automatically disconnecting the battery when thevoltage drops below a certain level.

Another object of the invention is to provide a novel emergency lightingsystem in which the converter output can be left unloaded withoutcausing any damage to the circuit.

A still further object of the invention is to provide a new and improvedcircuit which allows the battery to be replaced without danger toservice personnel.

SUMMARY OF THE INVENTION In a preferred embodiment of the invention atransistor switch is connected between the battery and a DC-AC converterwhich in turn energizes a fluorescent lamp load under emergencyconditions. A transistor control circuit senses the condition of the ACline voltage which, if present, causes the transistor switch todisconnect the battery from the converter. The transistor controlcircuit also functions, under emergency conditions, to connect thebattery to the converter via the transistor switch when it senses afailure of the AC line voltage. The transistor control circuit serves adual function in that it also senses the level of the battery voltageunder emergency conditions and operates the transistor switch todisconnect the battery from the converter when the battery voltage dropsbelow a predetermined safe level. A further feature of the invention isthe provision of circuitry to regulate the battery charge current sothat variations in the AC line voltage do not effect the charge current,whereby a charge current closely approaching the maximum batteryovercharge current can be supplied without danger to the battery.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be described ingreater detail with reference to the accompanying drawing, the soleFIGURE of which illustrates a schematic diagram of a preferredembodiment of the emergency lighting system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the FIGURE, inputterminals 1, 2 are adapted to be connected to the Hz AC supply linesnormally available from a local electric utility or the like. A testswitch 3 is connected between input terminal l and a terminal of theprimary winding of a supply transformer 4. The other terminal of thetransformer primary winding is directly connected to input terminal 2. Aconventional ballast device 5 is connected between the transformerprimary winding and a gas or vapor discharge lamp 6. As long as the ACpower is available at input terminals 1, 2, the discharge lamp will beenergized via the ballast 5 and test switch 3 in the conventionalmanner. A battery charger circuit for nickel cadmium battery 7 includesthe secondary winding 11 of transformer 4, diode 8, current limitingresistor 9 and diode 10 connected in series across the battery 7. Thebottom terminal of battery 7 and secondary winding 11 are each connectedto a common ground terminal for the system. Diode 10 serves to isolatethe automatic control circuit, to be discussed below. The battery ischarged with a half wave charging current by means of the diode 8. Alamp 12 is connected across resistor 9 to provide a visual indication ofthe operation of the charge circuit.

A resistor 13 is connected between the collector of NPN transistor T,and the junction point between resistor 9 and diode 10. The emitter oftransistor T is connected to ground via an emitter resistor 14. A zenerdiode 15 and a resistor 16 are serially connected between the junctionof diode 8 and resistor 9 and the base of transistor T Variations in thebattery charge current due to variations in the AC line input voltageare substantially eliminated by the regulating action provided bytransistor T in cooperation with resistors 13, 14 and 16 and zener diode15. Zener diode 15 provides level shifting and allows the base oftransistor T to follow variations in the voltage at the cathode of diode8. An increase in the AC line voltage causes a proportional increase inconduction through transistor T which tends to maintain the batterycharge current constant irrespective of the aforesaid variations in theAC input voltage. Similarly, a decrease in the AC line voltage causes aproportional decrease in the conduction of transistor T with acorresponding current regulating operation for the battery. As a resultof the action of T and its associated regulating elements, a chargecurrent closely approaching the maximum battery overcharge current canbe used in the system thereby improving the system efficiency andwithout danger to the battery.

In addition to the battery charging circuit already described, theemergency lighting system also includes an automatic control circuit anda DC-AC converter. The automatic control circuit controls the currentdrain from the battery. It includes means for sensing the AC line inputvoltage along with means for preventing the flow of battery current tothe converter in the presence of the AC line voltage and means forsupplying battery current to the converter in the absence of the AC linevoltage. It also prevents a reversal of the battery polarity byautomatically terminating the current drain from the battery when thebattery voltage drops below a given potential level.

The automatic control circuit includes a PNP transistor T having itsemitter connected directly to the positive terminal of battery 7 and itscollector connected to a center tap on the primary winding of theconverter transformer 17 via a lead.l8. The base of transistor T isconnected to the collector of NPN transistor T by means of a resistor19. The emitter of transistor T is connected directly to ground.Transistor T controls the supply of battery power to the converter andis in turn controlled by the conductive state of transistor T The baseof transistor T is connected to the collector of NPN transistor T and tothe cathode of diode 20. The emitter of T is connected to ground.Transistor T controls transistor T A voltage divider consisting ofserially connected resistors 21, 22 and 23 is provided between thecathode of diode 8 and ground. A capacitor 24 is connected between thejunction of resistors 21 and 22 and ground. In order to sense thepresence of an AC input voltage, the half wave rectified voltage at thecathode of diode 8 is converted into a DC voltage by means of theresistor 21 and capacitor 24. The DC voltage across capacitor 24 isapplied to the base of transistor T via a resistor 25. A portion of thecapacitor DC voltage is tapped off at the junction 30 of resistors 22and 23 and is directly coupled to the base of transistor T A filtercapacitor 26 is connected between the base of T and ground.

A second voltage divider consisting of the series combination ofresistors 27 and 28 is connected between the collector of transistor Tand ground. A second filter capacitor 29 is connected between the line18 and ground. The junction of resistors 27 and 28 is con- .nected tothe anode of diode 20.

In the presence of an AC input line voltage, the DC voltage at the tappoint 30 will drive transistor T into saturation. Saturation of T willcut-off transistor T and in turn transistor T As a result, there will beno battery power supplied to the converter via line 18. This is thenormal mode of operation of the system and power to the lamp 6 is thenderived from the AC input terminals via ballast 5.

In the case of a failure of the AC line input voltage, capacitor 24begins to discharge via resistors 22 and 23 and the transistor T At agiven voltage level across capacitor 24, transistor T, will start tocut-off while transistor T will begin to conduct. Conduction intransistor T allows transistor T to conduct. The battery voltage is thenapplied to resistors 27 and 28 and to capacitor 29 via line 18 andtransistor T The DC voltage developed across resistor 28 by the battery7 provides base drive to transistor T via diode 20 so that transistor Tis driven into saturation and is held in saturation even after capacitor24 completely discharges. Transistor T is also driven into saturationand provides the battery supply voltage for the converter via line 18.

If, after a period of time, the AC input voltage has not yet returned,the battery voltage will drop to a given level such that the voltagedrop across resistor 28 is no longer sufficient to maintain transistor Tin saturation. Transistor T will start to cut-off and so will transistorT thereby further reducing the base drive for transistor T The circuitrapidly latches up with transistors T and T completely cut-off whichthereby prevents any further discharge of the battery.

The DC-AC converter is an inductively coupled freerunning multivibratordesigned to provide a square wave output which reduces power losses inthe converter output transistors and thus improves the converterefficiency. It is of course obvious that other forms of oscillatorcircuits could be used in place of the free-running multivibrator. Theconverter includes NPN transistors T and T with their emitter electrodesconnected together to ground. The collector electrodes of T and T aredirectly connected to opposite end terminals of the primary winding ofoutput transformer 17. Feedback windings 31 and 32 of transformer 17each have one end terminal connected together to junction point 33. Theother end of winding 31 is connected to the base of transistor T and theother end of winding 32 is connected to the base of transistor T Thejunction point 33 is connected to ground through the parallel connectionof resistor 34 and capacitor 35 and to the DC supply line 18 via aresistor 36. Transistors T and T operate in the switching mode and theenergy required to operate them as switches is supplied by the feedbackwindings 31 and 32. The core of the output transformer 17 is driven intopositive and nega tive saturation on alternate half cycles and inducesin the transformer windings an alternating square wave voltage. Thesecondary winding of output transformer 17 delivers a square wavevoltage to the lamp load 6 via a series connected current limitingcapacitor 37. The converter operates to supply emergency lighting powerto lamp 6 whenever a DC voltage is supplied to line 1-8 from battery 7via transistor T as previously described.

As will be appreciated from the foregoing detailed circuit description,the present invention will achieve the various objects set forth in thepreamble. In addition, since the battery provides the bias voltage for Tvia the emitter-collector path of T once the battery supply to line 18is opened, the converter cannot start again unless the AC input voltageis once again applied to the system. This provides, inter alia, a safetyfeature when servicing the system. For example, if the AC power isturned off via switch 3 and the battery is replaced, the convertercannot come on again until the AC power is once again applied to thesystem and then turned off. This can be seen from the fact thatcapacitor 24 must be charged up before the described sequence ofoperations can begin. The automatic control circuit prevents excessivedischarge of the battery by cutting off transistor T and prevents anyfurther operation until the AC input voltage appears again. Otherembodiments of the invention and various modifications of the disclosedembodiment may occur to those skilled in the art without departing fromthe spirit and scope of the invention. Accordingly, it is to beunderstood that the invention is only limited in scope as defined in theappended claims.

I claim:

1. An emergency system for operating an electric load comprising, a pairof AC line voltage input terminals coupled to said load, a source of DCvoltage, an oscillator with its output coupled to said load, acontrolled switching device for selectively coupling said DC voltagesource to the oscillator to provide the operating voltage for theoscillator, circuit means for sensing the AC voltage across said inputterminals and including means coupled to the control electrode of saidswitching device to hold the switching device open when the AC voltageis present and to close the switching device in the absence of said ACvoltage, and means including the DC voltage source and the switchingdevice for supplying a bias voltage to said circuit sensing means of amagnitude and polarity to hold the switching device closed uponactuation thereof by the circuit sensing means and during the time theAC voltage is absent from the input terminals.

2. An emergency system as claimed in claim 1 wherein said circuitsensing means includes a second controlled switching device with acontrol electrode coupled to said DC voltage source via said firstswitching device so that a bias voltage is applied thereto when thefirst switching device is closed, means for coupling an output electrodeof the second switching device to the control electrode of the firstswitching device, and

means for effectively coupling the AC voltage at the input terminals tothe control electrode of 'the second switching device.

3. An emergency system as claimed in claim 2 wherein said circuitsensing means further comprises, means for rectifying the AC voltagepresent at the input terminals, a capacitor with acharge circuit coupledto the rectifying means, and means for applying the capacitor voltage tothe control electrode of the second switching device.

4. An emergency system as claimed in claim 3 wherein said DC voltagesource comprises a battery coupled to said rectifying means to receive acharge current therefrom, said system further comprising means forsensing the rectified input voltage and responsive thereto for varyingthe charge current so as to maintain said current constant despitevariations in the AC voltage.

5. An emergency system as claimed in claim 2 wherein said bias voltagesupplying means comprises a voltage divider connected to the DC voltagesource via the first switching device and having a tap point coupled tothe control electrode of the second switching device and located on thedivider so that at a predetermined lower level of the potential of theDC voltage source the second switching device will change state andthereby control the first switching device into its open conditionthereby to inhibit the supply of bias voltage and the oscillatoroperating voltage.

6. An emergency system as claimed in claim 5 wherein said DC voltagesource comprises a battery and the load comprises an electric dischargelamp.

7. An emergency lighting system comprising, an electric discharge lamp,a ballast device, a pair of AC voltage input terminals coupled to saidlamp via the ballast device, a battery, a rectifier connected betweenthe input terminals and the battery to supply a DC charge current to thebattery, a capacitor coupled to said rectifier to be charged thereby, anoscillator with its output coupled to said lamp, a first transistor withits emittercollector path connected between the battery and a DC voltagesupply line for the oscillator, a second transistor with an outputelectrode coupled to the base of the first transistor to control thecurrent flow therein, a third transistor with its emitter-collector pathin parallel with the base-emitter circuit of the second transistor,means for coupling the capacitor voltage to the base electrodes of thesecond and third transistors so as to drive the third transistor intosaturation when the AC voltage is present to charge the capacitor, saidsecond and first transistors then being held in cut-off by said thirdtransistor whereby operation of the oscillator is inhibited for lack ofa DC line supply voltage, a bias circuit for the base of the secondtransistor that is coupled to the battery via the emitter-collector pathof the first transistor, and a discharge circuit for said capacitoradapted to discharge same upon a failure of the AC input voltagewhereby, upon a given discharge of the capacitor, the third transistorreverts to the cut-off state to allow said second and first transistorsto turn on and supply via the first transistor the DC operating voltagefor the oscillator and a bias voltage for the base of the secondtransistor, said bias voltage being of a magnitude and polarity to holdsaid second and first transistors in conduction.

8. An emergency system as claimed in claim 1 wherein said circuitsensing means includes means responsive to the AC input voltage forholding the switching device open and for preventing said switchingdevice from closing unless AC voltage is present at the input terminalsand then subsequently is absent therefrom whereby operation of theoscillator is conditioned upon the appearance subsequent of the ACvoltage at the input terminals.

9. An emergency system as claimed in claim 1 wherein said means coupledto the switching device control electrode comprises a device jointlycontrolled by the DC voltage source via said switching device and by theAC voltage at the input terminals for controlling the operation of theswitching device so as to inhibit the start of oscillations of theoscillator unless the AC and DC voltages are both present.

10. An emergency system as claimed in claim 1 wherein said bias voltagesupplying means includes an impedance element responsive to the DCvoltage source in the closed condition of the switching device to applya bias voltage to said circuit sensing means that is determined by theamplitude of the DC voltage.

11. An emergency system as claimed in claim 1 wherein said circuitsensing means comprises a second controlled switching device having acontrol electrode coupled to said bias voltage supplying means and anoutput electrode coupled to the control electrode of the first switchingdevice, said DC voltage source comprises a battery, means coupled tosaid AC input terminals and said battery for applying a constant chargecurrent to said battery, and means for coupling the AC voltage at theinput terminals to the control electrode of the second switching device.

12. An emergency system as claimed in claim 11 further comprisingindicator means coupled to said charge current applying means, andwherein said bias voltage supplying means includes an impedance elementresponsive to the DC voltage source in the closed condition of theswitching device so that at a given voltage level of the DC voltagesource the second switching device will change state and thereby operatethe first switching device into its open condition to block the passageof the bias voltage and the oscillator operating voltage.

1. An emergency system for operating an electric load comprising, a pairof AC line voltage input terminals coupled to said load, a source of DCvoltage, an oscillator with its output coupled to said load, acontrolled switching device for selectively coupling said DC voltagesource to the oscillator to provide the operating voltage for theoscillator, circuit means for sensing the AC voltage across said inputterminals and including means coupled to the control electrode of saidswitching device to hold the switching device open when the AC voltageis present and to close the switching device in the absence of said ACvoltage, and means including the DC voltage source and the switchingdevice for supplying a bias voltage to said circuit sensing means of amagnitude and polarity to hold the switching device closed uponactuation thereof by the circuit sensing means and during the time theAC voltage is absent from the input terminals.
 2. An emergency system asclaimed in claim 1 wherein said circuit sensing means includes a secondcontrolled switching device with a control electrode coupled to said DCvoltage source via said first switching device so that a bias voltage isapplied thereto when the first switching device is closed, means forcoupling an output electrode of the second switching device to thecontrol electrode of the first switching device, and means foreffectively coupling the AC voltage at the input terminals to thecontrol electrode of the second switching device.
 3. An emergency systemas claimed in claim 2 wherein said circuit sensing means furthercomprises, means for rectifying the AC voltage present at the inputterminals, a capacitor with a charge circuit coupled to the rectifyingmeans, and means for applying the capacitor voltage to the controlelectrode of the second switching device.
 4. An emergency system asclaimed in claim 3 wherein said DC voltage source comprises a batterycoupled to said rectifying means to receive a charge current therefrom,said system further comprising means for sensing the rectified inputvoltage and responsive thereto for varying the charge current so as tomaintain said current constant despite variations in the AC voltage. 5.An emergency system as claimed in claim 2 wherein said bias voltagesupplying means comprises a voltage divider connected to the DC voltagesource via the first switching device and having a tap point coupled tothe control electrode of the second switching device and located on thedivider so that at a predetermined lower level of the potential of theDC voltage source the second switching device will change state andthereby control the first switching device into its open conditionthereby to inhibit the supply of bias voLtage and the oscillatoroperating voltage.
 6. An emergency system as claimed in claim 5 whereinsaid DC voltage source comprises a battery and the load comprises anelectric discharge lamp.
 7. An emergency lighting system comprising, anelectric discharge lamp, a ballast device, a pair of AC voltage inputterminals coupled to said lamp via the ballast device, a battery, arectifier connected between the input terminals and the battery tosupply a DC charge current to the battery, a capacitor coupled to saidrectifier to be charged thereby, an oscillator with its output coupledto said lamp, a first transistor with its emitter-collector pathconnected between the battery and a DC voltage supply line for theoscillator, a second transistor with an output electrode coupled to thebase of the first transistor to control the current flow therein, athird transistor with its emitter-collector path in parallel with thebase-emitter circuit of the second transistor, means for coupling thecapacitor voltage to the base electrodes of the second and thirdtransistors so as to drive the third transistor into saturation when theAC voltage is present to charge the capacitor, said second and firsttransistors then being held in cut-off by said third transistor wherebyoperation of the oscillator is inhibited for lack of a DC line supplyvoltage, a bias circuit for the base of the second transistor that iscoupled to the battery via the emitter-collector path of the firsttransistor, and a discharge circuit for said capacitor adapted todischarge same upon a failure of the AC input voltage whereby, upon agiven discharge of the capacitor, the third transistor reverts to thecut-off state to allow said second and first transistors to turn on andsupply via the first transistor the DC operating voltage for theoscillator and a bias voltage for the base of the second transistor,said bias voltage being of a magnitude and polarity to hold said secondand first transistors in conduction.
 8. An emergency system as claimedin claim 1 wherein said circuit sensing means includes means responsiveto the AC input voltage for holding the switching device open and forpreventing said switching device from closing unless AC voltage ispresent at the input terminals and then subsequently is absent therefromwhereby operation of the oscillator is conditioned upon the appearancesubsequent of the AC voltage at the input terminals.
 9. An emergencysystem as claimed in claim 1 wherein said means coupled to the switchingdevice control electrode comprises a device jointly controlled by the DCvoltage source via said switching device and by the AC voltage at theinput terminals for controlling the operation of the switching device soas to inhibit the start of oscillations of the oscillator unless the ACand DC voltages are both present.
 10. An emergency system as claimed inclaim 1 wherein said bias voltage supplying means includes an impedanceelement responsive to the DC voltage source in the closed condition ofthe switching device to apply a bias voltage to said circuit sensingmeans that is determined by the amplitude of the DC voltage.
 11. Anemergency system as claimed in claim 1 wherein said circuit sensingmeans comprises a second controlled switching device having a controlelectrode coupled to said bias voltage supplying means and an outputelectrode coupled to the control electrode of the first switchingdevice, said DC voltage source comprises a battery, means coupled tosaid AC input terminals and said battery for applying a constant chargecurrent to said battery, and means for coupling the AC voltage at theinput terminals to the control electrode of the second switching device.12. An emergency system as claimed in claim 11 further comprisingindicator means coupled to said charge current applying means, andwherein said bias voltage supplying means includes an impedance elementresponsive to the DC voltage source in the cLosed condition of theswitching device so that at a given voltage level of the DC voltagesource the second switching device will change state and thereby operatethe first switching device into its open condition to block the passageof the bias voltage and the oscillator operating voltage.